I missed this when it was updated, so I'm really late, but I called it. I said a few chapters ago when the beginning of the coup was revealed that he's either going to lose or walk away from the throne, because he knows that the demons stand no chance as long as humanity can conjure purification magic at-will. The MC is functionally a war-ender, and courting him is the single most productive thing the demon army could ever possibly do. With him as an ally, they can't lose, and with him as an enemy, not only is victory impossible, but their outright extinction is inevitable.
They seem to be going out of their way to set up the MC as his own faction, pulling people from both the demon and human side into a third party neutral faction with him at it's center, and it's been pretty obvious from the start that the Demon King was going to be one of those people in that faction. If that is indeed what's going to happen, then he could only ever either lose or forfeit the fight, and forfeiting makes more sense; he has zero reason to fight his own people. They will quickly find out on their own what he already knows if they seriously press the war without him. They'll find out exactly why they can't win. The hardliners and idiots will die, the reasonable and intelligent demons will come looking for the Demon King. Likewise, I assume when the asshole king regains consciousness from knocking himself out helping with the kingdom's own purification ritual, he will probably overreact to everything that's happened and exile his daughter, sending the princess right into the MC's faction as well.
The longer this series goes on, the more the asshole characters are isolating themselves and setting themselves up to fail. The MC is going to be the last one standing in the end.